In the past, soft contact lenses have been designed according to the corneal shape to afford maximum stability, fit and comfort for the wearer such that there is an optimum relationship between the corneal and/or scleral profiles and the back surface of the contact lens. The comfort has generally been achieved by not designing the back surface of a lens. Thus, correcting refractive error has been achieved by designing the front surface of the lens only.
The only exception to this design philosophy has been one form or another of back surface bifocal. This type of lens is of a specific nature and is designed only to correct presbyopia (a condition whereby the eye has some loss or reduction in its power of accommodation). Hence a lens can be made with two or more focal lengths by adding a number of curves to the back or front surfaces on the lens.
Designing contact lenses primarily by back surface comfort criteria has a number of attendant disadvantages which arise with the aim to gain close proximity between the lens and the corneal/scleral profile. One disadvantage is that all refractive error must be corrected from the front surface geometry alone, thus restricting the designer in his/her choice of anterior topography.
Also lens thickness is increased peripherally as the lens power increases. Designers can reduce the front optic zone to a certain extent in order to offset this unwanted thickness somewhat, but they are limited to how small they can go.
Furthermore in back surface toric designs the extension of the toricity out to or towards the circumference of the lens will result in thick and thin areas throughout the lens. This can result in uneven lid forces being exerted on the lens during the blinking phase and cause lens rotation.